The load in an elevator car of a traction elevator system has been used by the car and/or group supervisory control for such control strategy functions as controlling by-passing of hall calls, initiating system "down peak", initiating special floor features, such as convention floor strategy, and the like.
Unbalanced car load, ie., a load, or lack of load, which either causes the weight on the car side of the traction ropes to exceed the weight on the counterweight side, or vice versa, has been detected and used to improve car dynamics, such as for providing smoother car starts.
The drive related compensation signals, related to unbalanced load, and the supervisory signals, related to actual car load relative to rated car load, are usually independently obtained.
A common arrangement for obtaining drive related compensation signals includes resiliently mounting the drive brake, and obtaining a signal indicative of the direction of load unbalance, if any. Unbalanced brake torque, however, may indicate more than just passenger loading. For example, in some elevator installations, no compensation chains or cables are provided, even on relatively tall buildings, such as outside elevators on hotel walls. Thus, unbalanced torque reflects the unbalanced weight of the hoist cables, with the hoist cable weight compensation error being maximum at the travel limits of the elevator car. Even when compensation is provided for the weight of the hoist roping, compensation chains and cables are available in a limited number of sizes. Thus, the compensation for the hoist ropes and traveling cable will usually have an error, and the error changes with car location in the building hatch.
It would be desirable to be able to derive unbalanced load signals for the motor control loop, and passenger load signals for the supervisory control, from the unbalanced torque, if the passenger load signals can be accurately obtained, regardless of the amount of hoist cable compensation error, and regardless of car position in the associated building. Thus, car load weighting switches on the car, as well as the associated wiring, would be eliminated.